Laser assisted interstitial alloying for improved wear resistance

ABSTRACT

A method of enhancing wear resistance of a metallic substrate includes applying a coating of an interstitial element to a surface of a substrate. A laser beam is directed onto a localized area of the metallic substrate coated with the interstitial element locally raising a temperature of the metallic substrate to a temperature causing the interstitial element to diffuse into the substrate. A layer of alloy including the interstitial element is generated onto the localized area of the metallic substrate. A focal point of the laser beam is disposed at a location spaced from the surface of the substrate for optimizing a power density of the laser beam at the surface of the substrate. The coating of interstitial element not diffused into the substrate is removed exposing the layer of alloy including the interstitial element.

PRIOR APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 61/813,297, filed Apr. 18, 2013.

TECHNICAL FIELD

The present application relates generally toward an improved process forincreasing hardness of a soft metallic substrate. More specifically, thepresent invention relates toward the use of a laser to assistinterstitial alloying of a soft metallic substrate.

BACKGROUND

A dichotomy exists when selecting metallic substrate for use inindustrial processes or mechanical devices that are subject tofrictional forces. During a fabricating or forming process, it ispreferable to select a soft material for ease of forming. However, aselection of soft material substrates results in poor durability,particularly when the device is subject to frictional forces. Therefore,when durability of a mechanical device is desired, a hard metallicsubstrate is selected, which is problematic when fabricating or formingthe device.

Various attempts have been made to coat soft metallic substrates toimprove wear resistance and related material loss known to cause adversedimensional changes to the substrate. For example, plasma coatings andweld overlays have been used, but offer limited durability andsignificantly increase the cost of fabricating due to requisitepost-machining operations. Vapor deposition has also been used toincrease surface hardness. However, mechanical bonds between the coatingand the substrate are weak causing the coating to degrade or loseadhesion causing vapor deposition to be of limited use.

Diffusion of interstitial elements having higher a hardness value than asoft alloy substrate has been experimented with, but has not achievedsignificant industrial use. Various attempts to improve control over aninterstitial alloying have not proven affective. Therefore, it would bedesirable to provide an enhanced process for increasing a hardness of asubstrate by way of diffusion of an interstitial alloy.

SUMMARY

A method of enhancing wear resistance of a metallic substrate includesapplying a coating including an interstitial element to a surface of thesubstrate. A laser beam is directed onto a localized area of themetallic substrate coated with the interstitial element. The laser beamlocally raises a temperature of the metallic substrate to a temperaturecausing the interstitial element to diffuse into the substrate providinga layer of alloy including the interstitial element onto the localizedarea of the metallic substrate. A focal point of a laser beam ispositioned at a spaced location from the surface of the substrate tooptimize a power density of the laser beam at the surface of thesubstrate. The coating of the interstitial element not diffused into thesubstrate is removed exposing a layer of alloy including theinterstitial element.

The present inventive method provides an enhanced ability to controlexcitation of substrate molecules to control diffusion of interstitialelements into a soft metallic substrate. By controlling the focal pointrelative to the surface of the substrate an optimum energy beam andenergy configuration is achieved to enhance control over the diffusionprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated,as the same becomes better understood by reference to the followingdetail description when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 shows a metallic substrate;

FIG. 2 shows a metallic substrate with a localized application of acoating including an interstitial element;

FIG. 3 shows a laser heating a localized area of the soft metallicsubstrate having a coating including an interstitial element;

FIG. 4 shows an alternative method of locally raising a temperature ofthe soft metallic substrate;

FIG. 5 shows a cylindrical component being subject to the method of thepresent invention;

FIG. 6 shows a process of diffusing an inside of a tubular componentusing a galvanometer to redirect the laser beam of the presentapplication; and

FIG. 7 shows a chart of experimental hardness of a substrate beingsubject to the method of the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a metallic substrate in the form of a planarcomponent is generally shown at 10. The metallic substrate 10 iscontemplated to be formed from metals, such as, for example, varioussteels, nickel alloys, cobalt alloys, aluminum alloys, and copperalloys. It is anticipated that the substrate 10 is machined or formedinto a final shape through grinding, machining, or turning as is knownto those of skill in the art. The substrate 10 is contemplated by theinventor to be any substrate 10 subject to frictional or othermechanical forces known to degrade the geometry and function of thesubstrate 10.

Knives, mechanical parts, such as, for example, piston heads, otherinternal combustion elements and any metallic component subject to wearare all believed to be enhanced by the process of the present invention.After processing, the substrate 10, it is desirable to include a surfaceroughness having an Ra value of less than about 20 microns and an Rtvalue of less than about 100 microns. As set forth above, the partgeometry includes a flat knife blade, a rotary knife blade, an enginecylinder liner, or a piston ring for an engine. It should be understoodby those of ordinary skill in the art that any metallic substratesubject to durability requirements is included within the scope of thisinvention.

FIG. 2 shows the metallic substrate 10 having a coating 12 applied overan area of interest known to be subject to frictional forces. Thecoating includes an interstitial element having an atomic size known toallow diffusion into a lattice structure of an alloy. More specifically,the coating includes at least one of hydrogen, boron, carbon, ornitrogen. Additionally, combinations of these interstitial elements areincluded within the scope of this invention to further enhance wearresistance of the metallic substrate 10.

The coating 12 is applied either as a powder, or a liquid, in whichcase, a solvent is used to liquefy and suspend the interstitial elementof choice. The solvent is either water or organic, but is selected toflash from the surface of the substrate 10 without requiring significantamount of time or heat. When a liquid coating 12 is applied to thesubstrate 10, the substrate 10 is preheated in an oven to a temperatureof about 240° C. for about 20 minutes so that the substrate (orcomponent) receives a uniform temperature. It should be understood bythose of ordinary skill in the art that the temperature selected toflash the solvent from the coating 12 is below the melting temperatureof the substrate 10 alloy to prevent adversely affecting the dimensionalconfiguration of the component. After preheating, the component isremoved from an oven and a coating including carbon black powder isapplied, or other interstitial element, using an aerosol or atomizingspray method. The coating includes a uniform thickness over the surfacerequiring improved wear resistance. In the alternative, a tapecomprising an interstitial element is applied to an area of interestthat requires enhanced wear protection.

Referring now to FIG. 3, a laser 14 is shown projecting a laser beam 16(or energy beam) onto an area of interest 18 that has received a coating12 including an interstitial element. The laser comprises a CO₂ laser, adiode laser, a fiber optic laser, or any equivalent energy source,capable of directing the laser beam 16 to a localized area of interest18 of the substrate. The laser beam 16 defines a laser focal point 20that is located at a position spaced from the surface of the substrate10 determined to optimize the power density of the laser beam at thesurface of the substrate 10. For example, it is believed that locatingthe focal point on the surface of the substrate 10 or too close to thesurface of a substrate results in generating a cast iron surface knownnot to provide durable property achieved by proper diffusion of aninterstitial element. Therefore, the location of the focal point 20 ispredetermined to provide a proper amount of energy to excite the latticestructure of the substrate alloy material known to allow diffusion ofthe interstitial element to a proper depth.

In one embodiment, the laser beam is a divergent laser beam where thefocal point 20 is spaced above the surface 22 of the substrate 10. It iswithin the scope of the invention that the laser beam is a convergentlaser beam where the focal point 20 would be positioned below thesurface 22 of the substrate 10.

The surface 22 of the substrate 10 does not melt under optimumcircumstances. The avoidance of a eutectic reaction which would resultin the interstitial element reacting with the substrate 10 alloy isdesirable. The optimum effect of the laser (or energy) beam 16 on thesubstrate is to merely excite the molecular lattice of the substrate 10alloy. As such, an optimum laser power and speed is predetermined foreach application based upon the substrate alloy and the desired depth ofdiffusion of the interstitial element. In one embodiment, a CO₂ laserprovides an adequate amount of energy to the substrate 10. In otherembodiments, a diode laser is preferable. Additionally, the laser 14 ismodified to project an alternatively shaped laser beam 16 onto the areaof interest of the substrate 10. In some application, a rectangularshaped laser beam 16 is preferable, such as, for example a 12×1millimeter or 20×1 millimeter shape laser beam. Further applicationsmake use of a round spot of 2 millimeters or 4 millimeters diameter, oran oval shape. In this regard, a shaping nozzle 36 (FIG. 6) is used.

In some applications, rapid diffusion of the interstitial element intothe substrate 10 required a serpentine path 24 be established. This isbest represented in FIG. 4 where the laser beam zig zags to cover moresurface area than capable by a single pass across an area interest ofthe metallic substrate 10. An optimum path of travel is determined basedupon a level of energy required to diffuse the interstitial element intothe substrate 10, which will dictate a size of the laser beam 16 at thesurface 22 of the substrate 10. It should be understood by those ofordinary skill in the art that either the laser 14 or the substrate 10is movable so that the laser beam 16 provides an adequate amount ofexcitation energy to the substrate 10.

FIG. 5 shows the ability of the present inventive method to diffuse aninterstitial element into components having various three dimensionalconfigurations. In this instance, a cylindrical element, such as, forexample, a piston rotates relative to the laser beam 16 to provide asingle circumferential band 24 around an exterior surface 26 of thecomponent. It is contemplated by the inventor that either circular toolpath or rectangular tool path provides an adequate level of excitationenergy to the substrate 10.

To further control diffusion of the interstitial element, the laser 14interfaces with a computer aided design (CAD) data to adjust thelocation of the focal point of the laser beam 16 to maintain a constantdistance from the surface of a three dimensional substrate 10. The CADdata is used to direct the laser to either adjust a physical locationrelative to the substrate 10 or adjust the focal point 20 by way of acontroller (not shown). Alternatively, the substrate 10 is movedrelative to the laser 14 by the controller.

A still further embodiment is shown at FIG. 6 where interstitialdiffusion into a substrate 10 is desired on an interior surface 28 of atubular component 30. In this embodiment, a laser beam 32 is directedtoward a galvanometer-controlled mirror 34 to redirect the laser beam32. Once redirected, the laser beam 32 passes through a shaping nozzle36 directing the divergent beam 38 onto an area of interest 40 on theinner surface 28 of the tubular component 30.

Tests have shown that the diffusion of the interstitial element rangesbetween a depth of 30 microns and 500 microns. The table shown in FIG. 7provides the test results where significant hardness improvement isachieved up to 10 millimeters from an edge of a knife blade (not shown).In this example, 1018 steel was coated with carbon powder and subject toexcitation by way of a laser beam 16, 38 as explained above. Maximumhardness of around 900 VHS is achieved to 9 millimeters indicating thedensity of the interstitial carbides similar or equal to the density ofinterstitial carbides at the surface. Hardness requirements of a givenapplication are achieved by adjusting the strength and speed of thelaser treatment of the area of interest on the substrate 10. The rangeof depth from the knife edge where hardness drops from above 800 VHS tothat of the un-alloyed substrate, or in this example around 300 VHS isidentified as the transition zone. At 11 millimeters the hardness dropsthat of the unalloyed substrate.

Following treatment of the component, the surface 22 of the metallicsubstrate 10 is polished to remove interstitial element not diffusedinto the substrate 10. In one embodiment, the surface is cleaned andpolished with a diamond paste having 0.3 micron sized diamond particlesmixed into a kerosene solution. However, it should be understood bythose of ordinary skill in the art that alternative polishing methodswill suffice.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. The foregoing inventionhas been described in accordance with the relevant legal standards;thus, the description is exemplary rather than limiting in nature.Variations and modifications to the disclosed embodiment may becomeapparent to those skilled in the art and do come within the scope of theinvention. Accordingly, the scope of legal protection afforded thisinvention can only be determined by studying the following claims.

What is claimed is:
 1. A method of enhancing wear resistance of ametallic substrate, comprising the steps of: providing a metallicsubstrate; applying a coating including an interstitial element to asurface of the substrate; directing a laser beam onto a localized areaof the metallic substrate coated with the interstitial element therebylocally raising a temperature of the metallic substrate to a temperaturecausing the interstitial element to diffuse into the substrate providinga layer of alloy including the interstitial element onto the localizedarea of the metallic substrate; positioning a focal point of the laserbeam at a location spaced from the surface of the substrate foroptimizing a power density of the laser beam at the surface of thesubstrate; and removing the coating of interstitial element not diffusedinto the substrate thereby exposing the layer of alloy including theinterstitial element.
 2. The method set forth in claim 1, wherein saidstep of directing the laser beam is further defined by directing thelaser beam along a three dimensional surface of the metallic substrate.3. The method set forth in claim 2, wherein said step of directing thelaser beam along a three dimensional surface of the metallic substrateis further defined by directing said laser beam with computer datadefining a configuration of said metallic substrate.
 4. The method setforth in claim 1, wherein said step of applying a coating of aninterstitial element is further defined by providing a coatingcomprising at least one of hydrogen, boron, carbon, nitrogen, orcombinations thereof.
 5. The method set forth in claim 1, wherein saidstep of providing a metallic substrate is further defined by providingiron-alloys (steel), nickel-alloys, cobalt-alloys, aluminum-alloys, andcopper-alloys.
 6. The method set forth in claim 1, wherein said step ofproviding a metallic substrate is further defined by providing ametallic substrate with a surface roughness having an Ra value less thanabout 50 microns and an Rt less than about 100 microns.
 7. The methodset forth in claim 1, wherein said step of causing the interstitialelement to diffuse into the substrate providing a layer of alloyincluding the interstitial element is further defined by causing theinterstitial element to diffuse into the substrate to a depth of between30 microns and 500 microns.
 8. The method set forth in claim 7, furtherincluding controlling the depth of diffusion of the interstitial elementby adjusting power density and laser traverse speed of the laser beam.9. The method set forth in claim 1, wherein said step of applying acoating of an interstitial element to the substrate is further definedby applying a powdered interstitial element using an aerosol spray orapplying a tape of comprising the interstitial element to apredetermined location.
 10. The method set forth in claim 1, whereindirecting a laser beam onto a localized area of the metallic substrateis further defined by adjusting a shape of the laser beam projected ontothe localize area of the metallic substrate.
 11. The method set forth inclaim 1, wherein said step of directing a laser beam onto a localizedarea of the metallic substrate is further defined by providing a laserbeam comprising a CO₂ laser, a diode laser, a fiber optic laserdelivering a laser beam directly to the surface of the substrate, andequivalents thereof.
 12. The method set forth in claim 1, furtherincluding the step of heating the metallic substrate during or prior toapplying the coating of interstitial element for vaporizing solventdisposed in the coating of the interstitial element.
 13. The method setforth in claim 1, wherein said step of applying a coating ofinterstitial element is further defined by applying a coating includingthe interstitial element comprising a volatile solvent capable ofevaporating from the coating including the interstitial element whilethe substrate is disposed at ambient temperature.
 14. The method setforth in claim 1, wherein said step of directing a laser beam onto alocalized area of the metallic substrate is further defined by directinga divergent laser beam onto the metallic substrate.